Traffic in the morning rush hour is now slower than before Ken Livingstone brought in the congestion charge.

Transport for London figures also show that journey times during the day and evening peak in central London have increased, after initially falling.

Motorists across the capital will be concerned by the revelation, as the Mayor pledged that congestion charging would increase traffic speeds.

Until now, Mr Livingstone has been able to claim that the £8 levy had cut the jams and made journeys quicker.

But the latest figures show the average speed for driving to work in 2006-7 was 9.3mph, down from 9.9mph before the scheme began in 2003.

A dedicated TfL vehicle, fitted with equipment to measure distances and journey times, travelled along 7,000 routes in Greater London to work out the average speed at different times of day, including the morning rush hour. The slowdown is thought to be caused in part by extra traffic entering the central zone since the west London extension in February. More drivers are now entitled to claim the 90 per cent residents' discount to drive inside the zone and many are making the most of the 80p rate.

Meanwhile, new bus lanes and schemes such as pavement widening have squeezed available road space and there has been a rise in the number of green vehicles exempt from the charge. Motoring experts have warned that the congestion could worsen if Mr Livingstone brings in exemptions for band A and B vehicles including Ford Fiestas, Nissan Micras and Volkswagen Polos.

A spokesman for London First, which represents big business in the capital, said: "The original congestion charge was a bold move by the Mayor. Although it was never universally popular, the Mayor could always point to its main achievement - it reduced congestion and increased traffic speeds.

"These latest figures show that TfL's failure to co-ordinate roadworks and the negative impact of the western extension 'buy-one-zone-get-one-free' incentives have set speeds back to 2002 levels."

TfL claimed that congestion charging had reduced traffic in the original charging zone by 21 per cent - some 70,000 cars every day - but did not deny the average traffic speed figures, blaming the fall on roadworks. " Without the congestion charge, London would have ground to a halt by now," a spokesman said.

"However, last year we saw a near doubling of streetworks taking up the road space. Much of this work is urgently required - such as replacing leaking, Victorian water pipes. Transport for London has very limited powers to ensure that roadworks are co-ordinated so as to minimise their impact on traffic flow.

"Other major world cities invested heavily in public transport for decades, while London was starved. That is why strong measures such as congestion charging have been necessary. There are no short cuts as massive investment is required, but we have made real progress, and the benefits of congestion charging in London remain clear."

Geoff Pope, Lib-Dem transport spokesman on the London Assembly, said: "Congestion charging is a clunky system based on old technology. What we need is a modern solution using tag and beacon technology, which can vary the charge to reduce congestion at peak times.

"Local action on traffic hot spots is the way forward, not hiking up the charge, which will only hit public sector workers and small businesses."